In memory disk equipment used in data processing systems the disk drive positioning systems must be arranged to provide a true positioning of the record/read heads with respect to the data tracks on the disk. During use, changes in temperature cause changes in the dimensions of the mechanical mounting system for the disks and also changes in the dimensions of the disk recording surface, thereby producing undesired errors in the positioning of the head relative to the data tracks. For disks having data track spacing densities of less than 100 tracks per inch, position errors which occur because of temperature changes are not normally significant enough to cause undue concern. However, as the data track spacing density becomes greater than 100 tracks per inch, such position errors become more significant and at track densities of 200 tracks per inch require the use of techniques for compensating for the temperature changes which occur. Errors due to such temperature changes are further aggravated when using disk pack systems wherein the disk packs thereof must be readily interchangeable with one another for use on the same disk drive equipment.
One technique used by those in the art for avoiding such temperature problems has been to provide a sufficient wait time before use of the system when installing a disk or disk pack so that thermal equilibrium is ultimately achieved after one disk pack has been substituted for another. In many cases, however, such wait times are impractical or undesirable in the applications for which the disk system is being used and the system must be ready for relatively immediate use after changing disk packs, when, for example, a relatively hot disk pack is replaced by a relatively cold disk pack, or vice versa. In such a case the system cannot wait until temperature stabilization has occurred.
To overcome such problems, some prior art systems have included specially designed and relatively complex mechanical mounting mechanisms for the track counting device used in the system so as to compensate for position changes which occur from disk pack to disk pack. Such systems are not only relatively costly but also tend to be subject to mechanical difficulties giving rise to problems in reliability. Other prior art systems have suggested that a temperature sensor be utilized to measure the general temperature of the environment in which the base plate casting of the mounting mechanism for the disk pack is situated. The temperature thus monitured is then used to provide adjustments of the record/read head position in accordance therewith. However, such systems do not take into account the temperature in the vicinity of the disk pack itself so that corrections for temperature changes which occur from disk pack to disk pack, particularly where "hot" packs are interchanged with "cold" packs, and vice-versa, cannot be taken care of.
One prior art system which has been designed for remedying such a situation includes a pair of temperature sensing devices, such as thermistors, for monitoring the temperature both at the base plate casting and at the disk pack cartridge itself, the latter device sensing the air temperature at the cartridge. The monitored signals are amplified and the difference therebetween is applied to the available and conventional record/read head servo positioning loop as an additional temperature compensating voltage which is appropriately combined in the servo feedback path with the normal position feedback signal. An example of such a system is that sold under the designation Model D3000 Disk Memory Drive, made and sold by PERTEC Peripheral Equipment. While such a system tends to improve the position accuracy, two additional problems arise therein.
First of all, in such a temperature compensated position servo loop, operation of the overall record/read system normally begins at a fixed time period following turn-on of the position servo loop after a disk pack or cartridge has been installed. The fixed time period as initially designed into the system is selected on the assumption that, at the end thereof, the temperature has adequately stabilized and system operation can thereupon proceed.
However, it is found that in such systems at the end of such fixed time period the temperature has often not stabilized to a desirable level and an abnormally high temperature error still exists, particularly when a "cold" pack has been interchanged with a previously used "hot" pack. In such cases the overall operation of the position servo system tends to deteriorate and the desired positioning accuracy is lost. Moreover, in some cases where the temperature difference between the interchanged disk packs is not so severe, the temperature level stabilizes relatively rapidly and the system is ready for operation well before the end of such fixed time period and yet the system cannot be put into immediate operation. Accordingly, valuable operating time may be lost while awaiting the passage of the fixed time period before operation can begin.
Secondly, such prior art systems are designed so that the temperature compensating error voltage is inserted into the position servo loop immediately upon turn-on thereof. In cases where the record/read head position error is relatively large and where the temperature error is also relatively high, conventional servo loop designs may not be able to readily handle such abnormally high combined errors in order to bring the heads to the commanded position as rapidly and effectively as possible without a more complicated initial re-design of the servo loop for such purpose. Such a prior art system has, accordingly, not provided means for assuring that the temperature compensation errors are inserted into the servo loop system only when the record/read heads are brought to a position which is sufficiently close to the desired commanded position that the servo loop operates as fast and effectively as possible.